Polyolefin polymers are well known articles of commerce. The uses of polyolefins are many and well known to those of skill in the art. Polyolefins have many useful physical properties. However, in many applications polyolefins display unacceptable cold flow properties, that is, at room temperature or service temperature, they exhibit flow when subjected to low levels of stress for an extended period. Cold flow resistance is a property of importance in many polymer applications. Cold flow is defined as the permanent or non-recoverable deformation of a polymeric article in response to a force or stress (lower than the yield stress) applied for an extended time at a selected temperature. Different polymers will exhibit different resistances to cold flow.
Polypropylene homopolymers and copolymers have come into wide use. Over 5 million tons (4 million metric tons) of polypropylene are manufactured each year in the United States alone. Polypropylene has a wide range of commercial uses, from packaging films and sheeting to molded food containers and fibrous constructions employed in diapers and hospital drapes, wraps, and gowns.
There are several classes of polypropylene. One of these classes is statistical copolymers of propylene and other olefins, sometimes also known as random copolymers. In the past this class has tended to be represented largely by copolymers of propylene and ethylene, usually made using Ziegler-Natta catalysts. Copolymerization of higher alpha-olefins (HAO) (those alpha-olefins of 5 or greater carbon atoms) with propylene, using Ziegler-Natta catalysts has been problematic in the past due to the lower reactivity of these catalysts towards higher alpha-olefins. The Ziegler-Natta catalyzed ethylene copolymers have generally found use based on their substantially different properties when compared to Ziegler-Natta catalyzed polypropylene homopolymers. Broadly, the differences between Ziegler-Natta catalyzed homopolymers and propylene-ethylene copolymers are seen in such copolymer properties as lower melting point, greater flexibility, better clarity, slightly improved toughness, softness in products like nonwoven diaper cover stock and/or improved resistance to degradation when exposed to high energy radiation e.g. gamma-rays, ultraviolet, or electron-beam.
EP 0 495 099 A1 to Mitsui Petrochemical Industries discloses a method for polymerization of the propylene .alpha.-olefins utilizing metallocene-alumoxane catalyst systems. This document also discloses a propylene .alpha.-olefin copolymer where the propylene is present from 90-99 mole % and the .alpha.-olefin is present from 1-10 mole %. This document discloses that the propylene .alpha.-olefin copolymers would have a narrow molecular weight distribution (Mw/Mn), the copolymer would have a low melting point, and the copolymers have excellent softness. The document also discloses a straight line relationship between T.sub.m and propylene content, however, no distinction is drawn to the melting point depression effect of different .alpha.-olefins.
EP 0 538 749 A1 to Mitsubishi Petrochemical Co. discloses a propylene copolymer composition to produce a film having excellent low-temperature heat sealing, where the composition has 1 to 70 wt % of A and 30-99 wt % of B where:
A is a propylene-ethylene or .alpha.-olefin copolymer where the .alpha.-olefin has 4-20 carbon atoms and a M.sub.w /M.sub.n of not more than 3.
B is a propylene-ethylene or .alpha.-olefin copolymer where the .alpha.-olefin has 4-20 carbon atoms and a M.sub.w /M.sub.n of 3.5 to 10.
Copolymer A is polymerized by a metallocene catalyst system.
Copolymer B is polymerized by a Ziegler-type catalyst.
Substantially all examples utilize propylene-ethylene copolymers or propylene homopolymers.
EP 0 318 049 A1 to Ausimont discloses crystalline copolymers of propylene with minor portions of ethylene and/or .alpha.-olefins. The copolymers are disclosed to have very good mechanical properties. The copolymers are polymerized in the presence of methyalumoxanic compounds. The examples of this document show propylene-ethylene and propylene-butene-1 copolymers.
U.S. Pat. No. 5,188,885 to Kimberly Clark Corporation discloses a fabric laminate that is softer, stronger, more abrasion resistant and has reduced particle emissions compared to fabric laminates that are thermally spot bonded made from isotactic polypropylene. The fabric laminate has at least some layers formed from an olefin copolymer, terpolymer or blends of olefin polymers. Where the olefinic polymers have a crystallinity of less than 45%, preferably between 31-35%. It is disclosed that such a polymer has a broadened melt temperature range. In an embodiment a random propylene copolymer can be formed by copolymerizing 0.5 to 5 weight % of ethylene into a propylene backbone, preferred is 3 wt % ethylene. Further this document discloses that unless a melt temperature differential of about 10.degree. C.-40.degree. C. exists between the spun-bonded and melt-blown layers, bonding will not be optimum and strength will be reduced.
U.S. Pat. No. 5,213,881 to Kimberly Clark Corporation discloses a nonwoven web for use as a barrier layer in a fabric laminate where the nonwoven web has an average fiber diameter of from 1-3 microns and pore sizes in the range of 7 to 12 microns. Such a nonwoven web is obtained by forming a melt-blown web from a resin having a broad molecular weight distribution and having a high melt flow rate, where the resin is modified by addition of a small amount of peroxide prior to processing. Generally a polymer with a molecular weight distribution (M.sub.w /M.sub.n) of 4-4.5 and a melt flow rate of about 400 g/10 min at 230.degree. C. Using the peroxide disclosed the M.sub.w /M.sub.n is reduced to a range from 2.2 to 3.5 and the melt flow rate increases to a range of 800-5000 g/10 min at 230.degree. C. Disclosed as suitable polyolefins are polypropylenes, polyethylenes or other alpha-olefins polymerized with Ziegler-Natta catalyst technology.
Polymers such as polyamide and polyester exhibit cold flow resistance that is generally greater than the cold flow resistance of most polyolefins. This cold flow resistance enables polyamides, polyesters and other such thermoplastics to be used in fiber-type applications that are substantially foreclosed to polyolefins, such as carpet, apparel, and fiber yarns. In apparel, the cold flow resistance and "drape" or "hand" of polyolefins is generally deficient to that of other thermoplastics, for instance the above mentioned polyamide or polyester. The cold flow resistance of conventional (i.e. Ziegler-Natta catalyzed) polyolefins is noticeably defensive to the thermoplastics discussed above. If conventional polyolefins are used in applications such as apparel, the apparel would show deformation after normal body movement, leading to an undesirable baggy look. Similarly, the drapeability and strength of conventional polyolefin fabrics limits their use in medical drapes, because the combination of required "form fitting" of a drape over, for example surgical tray, requires good drape, while the same surgical tray will often have sharp edges requiring strength of the fabric to prevent punctures.
The less desirable drape or hand of polyolefins compared to polyamides or polyesters, gives a stiff or "boardy" look and feel to polyolefin fabrics and may impair functionality where drape or drapeability are important. These are unacceptable in general apparel applications where aesthetics are relatively important.
There is therefore a need for a polyolefin, specifically a polypropylene copolymer that will yield a soft, drapeable fabric and will resist cold flow to a sufficient extent that it could replace polyamides or polyesters in many fiber applications.